Umiko frequently shoots or flicks Shizuki's forehead whenever the latter changes the games specs on a whim. Umiko often searches for Shizuku when she needs something and the latter is not in her office.
She is very aware of Rin's feelings towards Kou. She likes the Rin/Kou tandem to the extent that she supports, teases and set the mood for the two.
Shizuku knows that Kou is unaware of Rin's feelings towards her, and often teases them both.
The two get along very well. Christina is one of the few people who has a higher authority than Shizuku. They're close enough to be drinking buddies. Shizuku is also the one who suggested she take Mozuku with her while visiting the character team booth.